1. Field of the Invention
The present invention relates to a hydrophobic catalyst layer for a polymer electrolyte fuel cell and a method of producing the same, and a polymer electrolyte fuel cell and a method of producing the same.
2. Description of the Related Art
A polymer electrolyte fuel cell is expected to be an energy generating apparatus in the future because the cell has high energy conversion efficiency, and is clean and quiet. Investigation has been recently conducted into the application of the polymer electrolyte fuel cell to not only a power source for an automobile, a domestic generator, or the like but also a power source for, for example, a small-size electrical apparatus such as a portable phone, a notebook personal computer, or a digital camera because the polymer electrolyte fuel cell has a high energy density and can operate at a low temperature. The polymer electrolyte fuel cell has been attracting attention because it may be driven for a long time period as compared to a conventional secondary battery.
The polymer electrolyte fuel cell has an advantage in that it can be driven even at an operating temperature of 100° C. or lower. On the other hand, the polymer electrolyte fuel cell has a problem in that the voltage of the cell gradually reduces with the lapse of an electricity generating time, and finally the cell stops generating electricity.
Such problem results from a so-called “flooding phenomenon” in which water produced as a result of a reaction resides in gaps of a catalyst layer, and water clogs the gaps in the catalyst layer to inhibit the supply of a fuel gas as a reactant, so that an electricity generation reaction is stopped. Flooding is apt to occur particularly in a catalyst layer on a cathode side where water is produced.
In addition, a reduction in size of the entire system is essential to the practical use of the polymer electrolyte fuel cell for a small-size electrical apparatus. In particular, in the case where the fuel cell is mounted on a small-size electrical apparatus, not only the size of the entire system but also the size of the cell itself must be reduced. Accordingly, a mode (air breathing) in which the air is supplied from an air hole to an air electrode through natural diffusion without the use of a pump, blower, or the like is considered to be promising.
In such case, the produced water is discharged to the outside of the fuel cell only by natural evaporation, so that the produced water resides in a catalyst layer to cause flooding in many cases. Accordingly, imparting hydrophobicity to the catalyst layer to improve the dissipation property of the produced water is considered to be an important factor on which the stability of the performance of the fuel cell depends.
A conventionally known method of making a catalyst layer hydrophobic involves mixing a fluorine resin-based fine particle powder made of polytetrafluoroethylene (PTFE) or the like as a hydrophobic agent with a solvent or a surfactant upon formation of the catalyst layer.
In addition, there have been proposed a method involving imparting the concentration distribution of hydrophobicity to the thickness direction of a catalyst layer to improve the dissipation property of produced water additionally (Japanese Patent No. 3245929), and a method involving making a part to which hydrophobicity is imparted maldistributed in the surface of a catalyst layer (Japanese Patent Application Laid-Open No. 2004-171847).
In addition, Japanese Patent Application Laid-Open No. 2001-76734 discloses a method of mixing fine particles composed of dimethylpolysiloxane in addition to fluorine-based resin fine particles. Japanese Patent Application Laid-Open No. 2001-76734 describes that the particle size of each of the hydrophobic fine particles is equal to that of a carbon carrier particle, and is preferably 10 μm or less.
Meanwhile, Japanese Patent Application Laid-Open No. 2006-49278 and Japanese-translated version's National Publication No. 2001-519594 each disclose a method of forming a catalyst layer for a fuel cell by means of a sputtering method or an ion plating method.
As described above, an approach of forming a catalyst layer by means of a vacuum film formation process such as a sputtering method has been recently developed. A conventional method involves: mixing catalyst particles, an electrolyte, and a solvent to prepare slurry; and mixing the slurry with hydrophobic particles to make the slurry hydrophobic. On the other hand, such production method as described in each of Japanese Patent Application Laid-Open No. 2006-49278 and Japanese-translated version's National Publication No. 2001-51959 does not involve mixing hydrophobic fine particles upon formation of a catalyst layer. That is, hydrophobicity is not imparted by means of a mixing method.